1. Field of the Invention
The present invention relates to a interdigital transducer for use in a transversal surface acoustic wave filter, a surface acoustic wave filter, and a radio communication apparatus.
2. Related Art of the Invention
As cellular phone terminals have become popular, their functions have been improved, while their sizes and power consumption have been reduced.
Under these circumstances, SAW (Surface Acoustic Wave) filters, characterized by consuming reduced power and having reduced sizes, have become essential for transmission and reception circuits of the cellular phone terminals.
FIG. 23 is a schematic view of a transversal SAW filter 407. The SAW filter 407 is mainly used for intermediate frequencies for a communication method such as CDMA.
The SAW filter 407 is composed of a piezoelectric substrate 401, an input IDT 402, an output IDT 403, an input terminal 404a, an input terminal 404b, an output terminal 405a, and an output terminal 405a. The IDT stands for an Interdigital Transducer.
Electric signals of intermediate frequencies input to the input terminals 404a, 404b as a pair of balanced terminals are converted into a surface acoustic wave by the input IDT 402. Then, the surface acoustic wave 406 propagates through the piezoelectric substrate 401 and is then converted into an electric signal again by the output IDT 403. The electric signal is output by the output terminals 405a, 405b as a pair of balanced terminals. In this manner, the SAW filter 407 operates as a transversal SAW filter.
FIG. 25 shows an IDT 1024 used as the input IDT 402.
The IDT 1024 is structured so that a plurality of electrode fingers are connected an upper bus bar electrode 224a and from a lower bus bar electrode 224b arranged opposite the upper bus bar electrode 224a. The upper bus bar electrode 224a and the lower bus bar electrode 224b are arranged parallel with a direction in which a surface acoustic wave propagates.
The IDT 1024 is composed of a section in which a surface acoustic wave is propagated in both directions and a section in which a surface acoustic wave is intensely propagated in one direction; the arrangement of the electrode fingers in the first section is different from that in the second section.
A bidirectional electrode 101 propagates a surface acoustic wave in two directions parallel with the upper bus bar electrode 224a and the lower bus bar electrode 224b. Further, a single phase unidirectional transducer (SPUDT) 102a located to the left of the bidirectional electrode 101 in the drawing intensely propagates a surface acoustic wave in a direction P in FIG. 25. Further, a single phase unidirectional transducer 102b located to the right of the bidirectional electrode 101 in the drawing intensely propagates a surface acoustic wave in the direction P in FIG. 25.
The bidirectional electrode 101 is structured so that a bidirectional electrode unit (a0)1 as a basic unit that propagates a surface acoustic wave in two directions parallel with the upper bus bar electrode 224a and a number of lower bus bar electrodes 224b are disposed at every distance equal to one wavelength of a surface acoustic wave at the predetermined frequency, and in FIG. 25, three successive bidirectional electrode units (a0) are disposed. The predetermined frequency of the surface acoustic wave is the center frequency of the surface acoustic wave excited on the piezoelectric substrate.
The bidirectional electrode units (a0)1 are each composed of four electrode fingers. The leftmost electrode finger in the drawing is connected the upper bus bar electrode 224a. The three other electrode fingers are connected the lower bus bar electrode 224b. 
These four electrodes each have a width equal to one eighth of the wavelength at the above mentioned predetermined frequency.
The single phase unidirectional transducers 102a and 102b are called xe2x80x9cEWC-SPUDT electrodesxe2x80x9d and utilize reflection of a surface acoustic wave therein to propagate this wave in one direction. These electrodes have hitherto been known as a method with a small loss. The single phase unidirectional transducer 102a is composed of a number of single phase unidirectional transducer units (a)2 as basic units which propagates a surface acoustic wave in the direction P and which are disposed at every distance equal to one wavelength at the predetermined frequency. Likewise, the single phase unidirectional transducer 102b is composed of a number of single phase unidirectional transducer units (a)2 as basic units which propagates a surface acoustic wave in the direction P and which are disposed every distance equal to one wavelength at the above mentioned predetermined frequency.
In FIG. 25, the single phase unidirectional transducer 102a is composed of two successive single phase unidirectional transducer units (a)2. The single phase unidirectional transducer 102b is also composed of two successive single phase unidirectional transducer units (a)2.
The single phase unidirectional transducer unit (a)2 is composed of three electrode fingers. The leftmost electrode finger in the drawing is connected the upper bus bar electrode 224a. The electrode finger located immediately to the right of the leftmost finger is connected the lower bus bar electrode 224a. The rightmost electrode finger in the drawing is connected the lower bus bar electrode 224b. Further, rightmost electrode finger is wider than the two other electrode fingers. For example, the rightmost electrode finger has a width equal to one-fourth of the wavelength at the predetermined frequency. The two other electrode fingers have a width equal to one-eighth of the wavelength at the predetermined frequency.
In this manner, the IDT 1024 is constructed so that the single phase unidirectional transducers 102a and 102b are arranged at the respective ends of the bidirectional electrode 101. For all basic units composed of the single phase unidirectional transducer units (a)2 and bidirectional electrode units (a0)1, the space between the excitation centers of adjacent basic units is a multiple of the above described wavelength. Accordingly, in the IDT 1024 as a whole, a surface acoustic wave is intensely propagated in the direction P in FIG. 25.
Accordingly, the IDT 1024 can be used as the input IDT 402 of the SAW filter 407 in FIG. 23 by connecting the input terminal 404a to the upper bus bar electrode 224a and connecting the input terminal 404b to the lower bus bar electrode 224b. 
Further, FIG. 26 shows an IDT 1025 used as the input IDT 402.
The IDT 1025 is structured so that single phase unidirectional transducers 112a and 112b are arranged at the respective sides of a bidirectional electrode 111. As with the IDT 1024 in FIG. 25, the bidirectional electrode 111 propagates a surface acoustic wave in two directions parallel with the upper bus bar electrode 224a and the lower bus bar electrode 224b. Further, both single phase unidirectional transducers 112a and 112b intensely propagate a surface acoustic wave in the direction P in FIG. 26 as with the IDT 1024 in FIG. 25.
The bidirectional electrode 111 is composed of three successive bidirectional electrode units (c0). The single phase unidirectional transducers 112a and 112b are each composed of two successive single phase unidirectional transducer units (c)12.
Bidirectional electrode units (c0)11 are each composed of four electrode fingers. The third electrode finger from the left in the drawing are connected the upper bus bar electrode 224a, and the other three electrode fingers are all connected the lower bus bar electrode 224b. Further, the single phase unidirectional transducer unit (c)12 is composed of three electrode fingers. The leftmost electrode finger is connected the lower bus bar electrode 224b. The second electrode finger from the left is connected the upper bus bar electrode 224a. The third electrode finger from the left is connected the lower bus bar electrode 224b. Thus, the IDT 1025 has an arrangement of electrode fingers, i.e. basic units different from those of the IDT 1024. However, for all basic units composed of the single phase unidirectional transducer units (c)12 and bidirectional electrode units (c0)11, the space between the excitation centers of adjacent basic units is a multiple of the above described wavelength. Accordingly, in the IDT 1025 as a whole, a surface acoustic wave can be intensely propagated in the direction P in FIG. 26. The other arrangements are similar to those in FIG. 25.
Thus, the IDT 1025 can be used as the input IDT 402 of the SAW filter 407 in FIG. 23 by connecting the input terminal 404a to the upper bus bar electrode 224a and connecting the input terminal 404b to the lower bus bar electrode 224b. 
However, in the bidirectional electrode unit (a)1, a basic unit of the bidirectional electrode 101 of the IDT 1024 in FIG. 25, one electrode finger is connected the upper bus bar electrode 224a, while three electrode fingers are connected the lower bus bar electrode 224b. Similarly, in the bidirectional electrode unit (c0)11, a basic unit of the bidirectional electrode 111 of the IDT 1025 in FIG. 26, one electrode finger is connected the upper bus bar electrode 224a, while three electrode fingers are connected the lower bus bar electrode 224b. 
Thus, in the bidirectional electrode 101 of the IDT 1024, the number of electrode fingers connected the upper bus bar electrode 224a is different from the number of electrode fingers connected the lower bus bar electrode 224b. Only one electrode finger contributes to excitation derived from the upper bus bar electrode 224a. Thus, the electrode is inefficiently excited. Consequently, the use of the IDT 1024 results in a large loss to the SAW filter 407.
Similarly, in the bidirectional electrode 111 of the IDT 1025, the number of electrode fingers connected the upper bus bar electrode 224a is different from the number of electrode fingers connected the lower bus bar electrode 224b. Only one electrode finger contributes to excitation derived from the upper bus bar electrode 224a. Thus, the electrode is inefficiently excited. Consequently, the use of the IDT 1025 results in a large loss to the SAW filter 407.
That is, in IDTs comprising a combination of bidirectional electrodes and single phase unidirectional transducers, of the single phase unidirectional transducers the number of electrode fingers connected an upper bus bar electrode is different from the number of electrode fingers connected a lower bus bar electrode. Disadvantageously, only one electrode finger contributes to excitation derived from the upper bus bar electrode. Thus, the electrode is inefficiently excited.
In view of these problems, it is an object of the present invention to provide an interdigital transducer that is efficient and undergoes a reduced loss, a surface acoustic wave filter, and a radio communication apparatus.
One aspect of the present invention is an interdigital transducer comprising:
a plurality of one-wavelength basic units of a single phase unidirectional transducer each having three electrode fingers within one wavelength; and
a plurality of one-wavelength basic units of a bidirectional electrode each having four electrode fingers within said one wavelength,
wherein said one-wavelength basic units are properly arranged according to a desired filter characteristic, and
one of the three electrode fingers of each one-wavelength basic unit of said single phase unidirectional transducer is wider than the two other electrode fingers, and
of the four electrode fingers of each one-wavelength basic unit of said bidirectional electrode, one pair of two fingers is interdigitated to the other pair of two fingers, and
an adjustment section is provided between the one-wavelength basic units of said single phase unidirectional transducer and the one-wavelength basic units of said bidirectional electrode.
Another aspect of the present invention is the interdigital transducer, wherein an excitation center of said at least one one-wavelength basic unit is substantially in phase with excitation centers of the other one-wavelength basic units.
Still another aspect of the present invention the interdigital transducer, wherein if N is an integer equal to or larger than 1, said adjustment means that said plurality of excitation centers are set at positions corresponding to values N times as large as said one wavelength if the excitation centers correspond to the electrode fingers on the same side bus bar electrode of the one-wavelength basic unit, and are set at positions corresponding to values (N-(1/2)) times as large as said one wavelength if the excitation centers correspond to the electrode fingers on the opposite side bus bar electrode of the one-wavelength basic unit.
Yet still another aspect of the present invention is the interdigital transducer, wherein provision of said adjustment section means that in an area where a one-wavelength basic unit of said single phase unidirectional transducer and a one-wavelength basic unit of said bidirectional electrode are adjacent to each other, the space between an adjacent-side end of the one-wavelength basic unit of the single phase unidirectional transducer and an adjacent-side end of the one-wavelength basic unit of the corresponding bidirectional electrode is. 2(Mxe2x88x921)/8 as large as said one wavelength, where M is an integer equal to or larger than 1.
Still yet another aspect of the present invention is the interdigital transducer, wherein said adjustment section has at least one electrode finger arranged therein.
A further aspect of the present invention is the interdigital transducer, further comprising at least two kinds of one-wavelength, basic units of said single phase unidirectional transducer,
the directivity of said one-wavelength basic units of single phase unidirectional transducer and the directivity of the other one-wavelength basic units of single phase unidirectional transducer are opposite directivity each other.
A still further aspect of the present invention is the interdigital transducer, further comprising:
an upper bus bar electrode from which some of said electrode fingers are connected; and
a lower bus bar electrode from which the other electrode fingers are connected,
wherein said single phase unidirectional transducer comprises at least two sections, and
wide electrode fingers of those one-wavelength basic units of said single phase unidirectional transducer which constitute one section of said single phase unidirectional transducer are connected one of said upper bus bar electrode and said lower bus bar electrode, and wide electrode fingers of those one-wavelength basic units of said single phase unidirectional transducer which constitute the other section of said single phase unidirectional transducer are connected the other of said upper bus bar electrode and said lower bus bar electrode.
A yet further aspect of the present invention is the interdigital transducer, wherein signals directly or indirectly input by a pair of balanced terminals are input to said upper bus bar electrode and lower bus bar electrode, or signals directly or indirectly output to the pair of balanced terminals by said upper bus bar electrode and lower bus bar electrode are output by said upper bus bar electrode and lower bus bar electrode.
A still yet further aspect of the present invention is an surface acoustic wave filter comprising:
a piezoelectric substrate;
an input electrode formed on said piezoelectric substrate; and
an output electrode formed on said piezoelectric substrate,
wherein the interdigital transducer is used in at least a part of said input electrode and said output electrode.
An additional aspect of the present invention is a radio communication apparatus comprising:
a transmission circuit that outputs a transmitted wave; and
a reception circuit that accepts a received wave as an input,
wherein said transmission circuit and/or said reception circuit comprises the surface acoustic wave filter.